Keto analogs of essential amino acids have been proposed for use in human nutrition as partial or complete substitutes for the corresponding amino acids, including, for example, leucine, isoleucine, methionine, phenylalanine, and valine. Originally, the use of such keto analogs was proposed by Dr. MacKenzie Walser as supplementation to protein-reduced diets in uremia. See, for example, Walser, et al., J. Clin. Inv. (1973) 52: 678-690. Further experiments by Walser and associates demonstrated a nitrogen sparing effect from mixtures of branched-chain keto acids. Saiper and Walser, Metabolism (1977) 26: 301-308. Patents have issued to Walser on the use of keto analogs of essential amino acids for promotion of protein synthesis and suppression of urea formation in humans. (U.S. Pat. Nos. 4,100,161 and 4,101,293). A recent review summarized existing knowledge with respect to the administration of branched-chain keto acids to humans. "New Aspects of Clinical Nutrition", pp. 319-324 (Karger, Basel, 1983).
The keto acid analog of L-leucine is alpha-ketoisocaproate (KIC) which is also sometimes referred to as "keto-leucine". KIC does not have L and D forms as does leucine. It is known that there is an interconversion of KIC and leucine in tissues. Published studies have demonstrated that KIC can be substituted in animal diets for leucine providing that larger molar amounts of KIC are used.
Chawla, et al. reported that weight loss by rats being fed a diet deficient in leucine could be prevented by adding equimolar amounts of KIC to the diet. J. Nutr. (1975) 105: 798-803. Other studies in rats have demonstrated that KIC is utilized less efficiently than leucine. Boebel et al. reported that the efficiency of KIC was only about 56% with reference to leucine. Boebel and Baker, J. Nutr. (1982) 112: 1929-1939. Chow et al. obtained similar results, reporting that substitution of KIC for leucine reduced feed efficiency by approximately 33%. Chow and Walser, J. Nutr. (1974) 104: 1208-1214.
Walser and associates have published a study of the oral dosing of KIC to rats in which an increased efficiency of utilization of nitrogen and minerals was apparently observed. Abras and Walser, Am. J. Clin. Nutr. (1982) 36: 154-161. Prior to the present invention, there are no known reports describing the feeding of KIC to domestic animals.
KIC has not been heretofore known as an immunomodulator in humans or animals, nor has it been suggested that KIC can be used to reduce plasma cortisol (hydrocortisone), or that it can be used to favorably alter immune function by enhancing blastogenesis of T lymphocytes.
It is known that in cattle stress-induced immunosupression is accompanied by increased plasma cortisol concentrations. Roth, Chap. 13, pages 225-243, in "Animal Stress" (published by Amer. Physiol. Soc., 1985). Such stress-induced immunosuppression can occur in other domestic animals. It can result from a wide variety of stresses, such as handling, shipping, injury, etc. However, this problem is particularly acute with cattle. Bovine respiratory disease complex, commonly known as "shipping fever", is recognized as a stress-associated condition. See Roth, above cited, pages 226-227. Considerable research has been devoted to finding ways of combating cattle shipping fever. See, for example, Kaeberle, et al., Immunopharmacology (1984), 8: 129-136; and Roth, et al., Immunopharmacology (1984), 8: 121-128.
It is known that T lymphocytes have important functions in immunological defense against microorganisms. See, for example, Williams et al., "Hematology", Chap. 102, pages 927-928 (2nd Ed., 1977, McGraw-Hill). In their immune function, T lymphocytes, which are normally quiescent, undergo mitosis with accelerated DNA production. Williams, et al., above cited, pages 882-884. This process of T lymphocyte activation is known as blastogenesis. The degree of blastogenesis activation of T lymphocytes can be determined in vitro using mitogens. The most commonly employed mitogens for this purpose are phytohemagglutin (PHA), pokeweed mitogen (PWM), and concanavalin A (conA). Williams et al., cited above, pages 895, and 923.